2. Field of the Invention
The present invention relates in general to apparatus for low pressure chemical vapor deposition (LPCVD)and, more particularly, to an improved structure in such LPCVD apparatus for depositing excellent quality of compound thin layers on wafers and for achieving the desired high integration degree of the semiconductor devices.
2. Description of the Prior Art
As well known to those skilled in the art, the low pressure chemical vapor deposition (LPCVD) technique has been wide used for deposition of insulating layers, dielectric layers, conductive layers and the like in production of semiconductor devices.
In the typical LPCVD, compound source gases such as SiH.sub.4, Si.sub.2 H.sub.6, TEOS, PH.sub.3, NH.sub.3, N.sub.2 O, TMOB, TMOP, O.sub.2 and N.sub.2 are thermally decomposed by heating them in vacuum and, thereafter, deposited on the wafers.
However, the above compound source gases are different in their thermal decomposition temperatures from each other so that there is a problem in management for retaining the optimal condition of vapor deposition in accordance with temperatures. Furthermore, since the vapor deposition temperatures of the compound thin layers are very high so that there is generated thermal shock between the thin layers deposited on the wafers and this changes the physical performances of the wafers and deteriorates the electric performances of the wafers. In this regard, several problems are caused in high integration and mass production of the semiconductor devices.
In the typical LPCVD, the compositions of compound thin layers deposited on the wafers are different from each other in accordance with locations of the wafers in the reactor. Therefore, several problems may be caused in the vapor deposition process as well as in the continued processes in production of semiconductor devices.
Another problem of the typical LPCVD is resided in that it may cause environmental contamination in its chemical reaction since the desired condition of chemical reaction is scarcely achieved.
Hereinbelow, construction and operation of typical LPCVD apparatus will be described in conjunction with the accompanying drawings, FIGS. 15 and 16.
FIG. 15 shows in a sectional view an example of typical LPCVD apparatus and FIG. 16 shows the LPCVD apparatus of FIG. 15 from which the boat is separated. As shown in these drawings, the LPCVD apparatus comprises a deposition base 1 which has a compound source gas inlet 1a and a reaction product outlet 1b. The inlet 1a and the outlet 1b extend from opposed side walls of the base 1. The apparatus further includes a reactor 2 of the double tube structure comprising an inside quartz tube 2a and an outside quartz tube 2b. The outside quartz tube 2b is airtightly coupled to the above deposition base 1 while the inside quartz tube 2a is placed in the outside tube 2b and defines a deposition reacting space S therein. The deposition reacting space S communicates with the compound source gas inlet 1a. There is provided a reaction product discharge path 2c between the inside and outside quartz tubes 2a and 2b and communicates with the reaction product outlet 1b of the deposition base 1. The bottom opening of the deposition base 1 is closed by a closing plate 3. A boat 4 is placed on the closing plate 3 and supports a plurality of wafers W thereon. The double structure reactor 2 is surrounded by reactor heating means 5. This heating means 5 is adapted for heating the reactor 2.
In the drawings, the reference numeral 1c denotes an annular support inwardly extending from an inner surface of the base 1, the numeral 6 denotes a deposition apparatus housing, the numeral 7 denotes a fixing bolt for fixing the deposition base 1 to the housing 6 and the numeral 8 denotes a lifting ram for lifting the boat 4. The boat lifting ram 8 is provided with a boat support 8a on its top end.
In deposition of chemical thin layers on the wafers W using the above LPCVD apparatus, the boat 4 is separated from the apparatus so as to seat the wafers W on the boat 4. After seating the wafers W on the boat 4, the boat 4 is inserted into the deposition reacting space S of the reactor 2 by operation of the boat lifting ram 8 as shown in FIG. 15. At the same time of insertion of the boat 4 with the wafers W, the closing plate 3 closes the bottom opening of the base 1. Thereafter, the compound source gas is introduced into the deposition reacting space S of the reactor 2 through the compound source gas inlet 1a of the base 1 while maintaining, using a vacuum equipment (not shown) and the heating means 5, both degree of vacuum and temperature of the space S suitable for vapor deposition process.
The compound source gas introduced into the lower section of the reactor 2 is in turn flows upward through the lower section of the inside quartz tube 2a. During the upward flowing of the source gas in the reactor 2, the gas is thermally decomposed and deposition-reacted so that the compound thin layers are deposited on the wafers W respectively.
At this time, the reaction products of the above chemical thin layer depositing reaction are exhausted to the outside of the apparatus through the discharge path 2c, defined between the inside and outside tubes 2a and 2b of the reactor 2, and the outlets 1b of the base 1 owing to the absorption force of the vacuum equipment (not shown).
However in the above typical LPCVD apparatus, the cool compound source gas is directly introduced into the lower section of the inside quartz tube 2a of the reactor 2 so that the thin layer depositing reaction of the cool gas at about the wafers W placed at the lower section of the reactor 2 is carried out even though the gas is not sufficiently heated and activated. This causes unstable deposition of the thin layers such as resulting in uneven doping effect. In addition, both the thickness and the composition of the thin layers deposited on the wafers W are not uniform in accordance with the positions of the wafers W.
The compound source gas is lifted through the inside tube 2a of the reactor 2 and, thereafter, flows at the upper section of the reactor 2 through the discharge path 2c formed between the inside and outside tubes 2a and 2b. In this regard, the heat of the heating means 5 is first transferred to the discharged compound source gas and, thereafter, transferred to the inlet compound source gas, so that the heating effect of the heating means 5 is deteriorated. In addition, the thin layers are deposited on the wafers W as well as on the inside and outside tubes 2a and 2b of the reactor 2. The repeated deposition of thin layers on the inside and outside tubes 2a and 2b shields the radiant heat of the heating means 5, so that the heating effect of the heating means 5 becomes worse. Therefore, the reactor 2 having the inside and outside quartz tubes 2a and 2b wastes large amount of power due to deterioration of the heating efficiency of the heating means 5 and is difficult to control the temperature. Another problem of the above typical reactor 2 is resided in that it causes somewhat difficulty in management of the inside and outside quartz tubes 2a and 2b.
Meanwhile when a native oxide is formed on each wafer, the chemical and physical characteristics of the chemical thin layers of the wafers W are changed so that it should be required to prevent forming of the above native oxide on the wafers so as to achieve the deposition of excellent chemical thin layers on the wafers W.
In order to achieve the above object, oxygen or an oxidant is removed and the boat 4 is moved with respect to the reactor 2 under the condition of inert atmosphere or conventional N.sub.2 atmosphere.
However in the typical apparatus, the N.sub.2 gas or the inert gas is introduced from the lower section of the inside tube 2a to the upper section of the tube 2a under the condition that the bottom of reactor 2 is opened by lowering the closing plate 3 and the boat 4 with the wafers W is removed from the reactor 2. During the above introduction of the N.sub.2 gas, there is generated an upward air current and, in this regard, the oxygen or the oxidant is introduced into the reactor 2 along with the upward air current. The oxygen introduced into the reactor 2 and the high inside temperature of the reactor 2 form the oxide on the wafers W during upward movement of the boat 4 with the wafers W toward the depositing position of the reactor 2, thus to deteriorate the characteristics of the chemical thin layers of the wafers W.
As described above, the chemical thin layers are deposited on the wafers W as well as on the inside and outside quartz tubes 2a and 2b of the reactor 2 during the thin typical layer deposition. With the thin layer deposition on the quartz tubes 2a and 2b, the tubes 2a and 2b needs separating from the base 1 and washing so as to remove the deposited material or needs substituting with new tubes.
However in the typical apparatus, the base 1 is fixed to the housing 6 by the fixing bolts 7 and the quartz tubes 2a and 2b are simply laid on the base 1 so that the separation and assembling of the quartz tubes 2a and 2b with respect to the base 1 are attended with waste of much time. Furthermore, the separation and assembling of the tubes 2a and 2b is a burdensome work.